1. Field of the Invention
The invention relates to a method and apparatus for on-site production of nitrate ions and bringing the nitrate ions into contact with an aqueous solution, particularly in an aqueous system used in oil-field applications.
2. Background of the Invention
Primary oil recovery generally yields less than 50% of what a given geological structure or reservoir contains. Accordingly, water injection is employed to enhance oil recovery from the porous rock formations that comprise many subterranean oil reservoirs. In enhanced oil recovery, an aqueous system is used to inject a water solution into an oil reservoir. An aqueous system can include above-ground facilities equipped with an apparatus or some facility that collects or distributes aqueous solutions, such as oil wells, oil water separators, water storage tanks, pipelines, and injection wells. The injection process is known to produce hydrogen sulfide (H2S), however, which sours oil reservoirs.
The hydrogen sulfide is produced by sulfate-reducing bacteria, which convert sulfate in the system to sulfide. Such bacteria can arise during the drilling for oil, but they also may be present indigenously, before the drilling. These bacteria and their affect on oil fields are described, for example, by J. R. Postgate, THE SULPHATE-REDUCING BACTERIA 2nd ed. (Cambridge University Press, 1984).
The hydrogen sulfide thus evolved causes corrosion of the equipment used to recover the oil and can drastically damage the production capabilities of the oil field and lowers the commercial value of the recovered crude oil. Accordingly, there has been intensive investigation directed at preventing the formation of hydrogen sulfide and/or removing the hydrogen sulfide once it is produced in oil fields.
It is known, for example, that the addition of molybdates will inhibit and/or kill the sulfate-reducing bacteria (SRB), which are responsible for the production of hydrogen sulfide in natural environments, such as sediments. Nevertheless, this method requires that vast amounts of molybdates, e.g., in excess of 3,000 ppm in the water to be treated, be used to effectively control the hydrogen sulfide production by SRB. The use of such large amount of molybdates has the associated disadvantages of high cost, due to the limited availability of molybdates and lower efficiency in saline environments or in other brine environments such as connate waters.
It also is known that the addition of nitrate ions to a system containing SRB will reduce the amount of SRB in the system and thus the amount of hydrogen sulfide formed by SRB. This method relies on strains of Thiobacillus denitrificans and other denitrifying microorganisms that are present in oil field waters. For example, hydrogen sulfide present in an aqueous system is removed and the production of hydrogen sulfide by sulfate-reducing bacteria is eliminated by introducing into the system nitrate and nitrate compounds and/or molybdate ions, whereby denitrifying microorganisms out-compete the sulfate-reducing bacteria for the available carbon nutrients, which prevents the SRB from producing hydrogen sulfide.
Nitrate for this purpose is typically formed by oxidizing ammonia or mined by conventional practice, and the resultant nitrate is transported and stored in close proximity to an oil field or other, often remote site for use. Transporting and storing large quantities of nitrate, or mixtures of nitrate with other solutions raises numerous safety and cost issues.
Accordingly, there is a need to provide an economical and effective means to locally produce nitrate ions and bring nitrate ions into contact with an aqueous system in order to prevent the formation of hydrogen sulfide in aqueous systems and remove any existing hydrogen sulfide in the system. Further, there is a need to provide a means to locally produce nitrate ions and bring nitrate ions into contact with an aqueous system which is useful in the recovery of oil which contains a reduced amount of hydrogen sulfide and furthermore will not be susceptible to forming hydrogen sulfide at a point later in the process, so that the system will not adversely affect the equipment used in the process. These and other needs have been solved by the present invention.